The Blues played just their fourth pre-season friendly on Wednesday evening, as they slipped to a 3-1 reverse to Celta Vigo at Tranmere Rovers' Prenton Park.
Roberto Martinez's side have also played against Porto, Leicester City and Tranmere, and they have yet to record a victory.
The Toffees conclude their pre-season preparations on Saturday against Paderborn, and the Blues will have played two games less than most of the Premier League rivals ahead of the new season.
Martinez has given his World Cup contingent specialised workout programmes in an attempt to bring them up to speed, with the Brazil tournament leading to some players requiring extra rest than the Spanish boss would have ideally wished.
However, Jagielka feels that the squad will be ready for the new season, after they underwent their intensive fitness work earlier this summer at a warm-weather training camp in Austria, although the concerns are something that the skipper has acknowledged.
The England international told the Liverpool Echo: “It’s always difficult. I’ve watched when lads who came back a bit later from previous World Cups and it always is hard to get the miles in.
“It’s going to be a really strange start to the season because there’ll be a lot of press saying the lads won’t be ready or fit to do 90 minutes.
“I don’t think you will be exactly where you want to be. But the fact of the matter is that it’s a fine line between bringing you back too early and you’re not getting enough rest; meaning you’re fine for the first two months of the season but by the time December comes you’re mentally and physically useless.”
The former Sheffield United star has also shrugged off the fact that his side are yet to win a game during pre-season.
“At the moment I feel good," he added. "I didn’t have my best game against Porto in the first 45 minutes but it’s not really going to affect my thinking.
“It’s part and parcel of pre-season – I haven’t won too many trophies in pre-season. It’s just a case of getting through it, and maybe judge us after the Leicester, Arsenal and Chelsea games and seeing how the lads look then.”
Everton have so far added Peter Henen to their ranks this summer, while also making the loan deals for Gareth Barry and Romelu Lukaku permanent.
Barry arrived on a free from Manchester City, while the Toffees smashed their transfer record to land Belgium international Lukaku from Chelsea for £28 million.